It is well known to route information, for example in tdm (time division multiplexed) channels, through one or more switches in order to establish communications between terminal devices such as telephones and data terminals. Each switch typically includes a data memory, a connection memory which serves to store addresses representing connections established via the data memory, and connection control means for controlling the contents of the connection memory and thereby controlling the setting-up and taking-down of connections via the switch.
The use of such a connection control means necessitates the provision of a connection control bus, in addition to incoming and outgoing data busses for the information being switched, complicating switch wiring and making integrated circuit fabrication of the switch difficult because of the large number of interconnections which are required. In addition, the use of the connection control means results in a considerable overhead and delay in establishing and terminating connections, as a result of which the switching of short messages, and the switching of switch control information in the same manner as data, is inefficient or impractical. Furthermore, the connection control means can become overloaded or saturated, so that it is not capable of establishing connections even though paths for such connections are available through the switch.
Thus there are distinct problems associated with the use of such a connection control means. A further problem with switches is that of blocking, in which no free path exists through a switch between an incoming channel and an outgoing channel which is free and to which the incoming channel is to be connected. It is known to avoid the problem of blocking by using single stage time switches which are inherently non-blocking, and to use an array or matrix of single stage time switches, each constituting a crosspoint node, to provide both space and time switching of channels. However, the use of a matrix of crosspoint nodes exacerbates the problem of providing interconnections to the connection control means.
An object of this invention, therefore, is to provide an improved method of switching information, and to provide an improved switch for establishing connections between incoming and outgoing tdm channels, which reduces or eliminates at least some of the above problems associated with known switching methods and switches.